1. Field of the Invention
This invention relates to a luminance signal-color signal separating circuit for color television receivers and the like.
2. Description of the Related Art
FIG. 13 is a block diagram showing a related art of a luminance signal-color signal separating circuit.
A composite video signal input through an input terminal 1 is delayed by 1H (H represents a horizontal period) in a 1H delaying circuit 2 and is further delayed by 1H in a 1H delaying circuit 3. Thus, the present signal a, 1H delayed signal b and 2H delayed signal c are given respectively to band pass filters (mentioned as BPF's hereinafter) 4, 5 and 6. The BPF's 4, 5 and 6 pass signal components of the frequency band (3.58 MH.sub.z band) near the color subcarrier frequency among the signals input into the BPF's 4, 5 and 6. The signals d and f having passed through the BPF's 4 and 6 are given respectively to amplifiers 11 and 12. The signal e having passed through the BPF 5 is inverted in the polarity by an inverting amplifier 7. The outputs of these amplifier 11, inverting amplifier 7 and amplifier 12 are given to a middle value detecting circuit 10.
FIG. 16 is a block diagram showing a concrete formation of the middle value detecting circuit 10.
The outputs of the amplifiers 11 and 12 are given respectively to input terminals 91 and 92. The output of the inverting amplifier 7 is given to an input terminal 93. A maximum value circuit (mentioned as an MAX hereinafter) 94 determines the maximum value of the signals input through the input terminals 91 and 93. An MAX 96 determines the maximum value of the signals input through the input terminals 91 and 92. A minimum value circuit (mentioned as an MIN hereinafter) 97 determines the minimum value of the outputs of the MAX's 94, 95 and 96. Thereby, the middle value of the signals input into the input terminals 91, 92 and 93 is output from the MIN 97. By the way, the MAX and MIN may be replaced with each other.
Thus, the middle value of the outputs of the amplifiers 11 and 12 and inverting amplifier 7 is detected. The output signals of the amplifiers 11 and 12 and inverting amplifier 7 are of the same phase on the carrier color signal component of the same horizontal period. Therefore, a middle value signal h containing the carrier color signal component will be output from the middle value detecting circuit 10. Also, the luminance signal component will be contained in the middle value signal only in the period when the luminance signal component is contained in any of the outputs of the amplifiers 11 and 12 and inverting amplifier 7.
The outputs of the amplifier 11 and inverting amplifier 7 are input into an adder 8, are added together and are given as an added signal g to a middle value detecting circuit 15 through an amplifier 13. Also, the outputs of the inverting amplifier 7 and amplifier 12 are input into an adder 9, are added together and are output as an added signal i to a middle value detecting circuit 15 through an amplifier 14. The middle value detecting circuit 15 is also of the same formation as of the middle value detecting circuit 10 and detects the middle value of the middle value signal h from the middle value detecting circuit 10 and the added signals g and i. As the outputs of the amplifier 12 and inverting amplifier 7 are of the phases reverse to each other in the luminance signal component in the same horizontal period, the luminance signal component will not be contained in the added signal g in the period when the luminance signal is contained in any of the outputs of the amplifiers 11 and 12 and inverting amplifier 7. Also, in the same manner, the luminance signal component will not be contained in the added signal i in this period. Therefore, the luminance signal component of the middle value signal h will be deleted by the middle value detecting circuit 15 and the middle value signal j output to the output terminal 18 from the middle value detecting circuit 15 will be a carrier color signal containing no luminance signal component.
The adder 16 adds together the middle value signal j and the output of the 1H delaying circuit 2. As the phases of the carrier color signals of both are reverse to each other, no carrier color signal component will be contained in the output signal 1 from the adder 16. Thus the luminance signal is led out of the output terminal 17. Thus, the luminance signal and color signal will be positively separated from each other and a picture comparatively high in the resolution even in the vertical direction will be able to be obtained.
Further, the circuit of FIG. 13 shall be explained as divided into the operations of a horizontal filter and vertical filter. The horizontal filter is formed of the BPF's 4, 5 and 6. An input composite video signal first has the color signal component in the horizontal direction extracted by this horizontal filter. On the other hand, a three-line dynamic comb filter (mentioned as a DCF hereinafter) circuit is adopted as a vertical filter. In FIG. 13, the DCF circuit is formed of the 1H delaying circuits 2 and 3 and a one-point chain line part 19. After the color signal component in the horizontal direction is extracted by the horizontal filter, the color signal component j in the vertical direction is extracted by the DCF circuit. Then, the luminance signal 1 is obtained by subtracting the thus obtained color signal j from the input composite video signal. By the way, the center frequency (f.sub.sc) of the color signal is given by the below mentioned formula (1) in the two-dimensional frequency expression: EQU .mu.=3.58 MH.sub.z, .upsilon.=525/4cpH (1)
wherein .mu. represents a horizontal frequency and .upsilon. represents a vertical frequency.
When the DCF circuit is adopted as a vertical filter, the dot obstruction generated in the part of a picture in which the color signal varies in the vertical direction will be able to be reduced. Thus, as compared with the case of using a comb filter utilizing the correlation between two horizontal lines, the precision of separating a luminance signal and color signal from each other in a vertical non-correlated part (mentioned as a Y/C separation precision hereinafter) can be improved to be higher.
However, there has been a problem that, as the oblique component of the luminance signal can not be judged, in a video image containing many oblique components, the Y/C separation will not be positively made and a cross color will be produced. This problem shall be explained with reference to the below mentioned Tables 1 and 2 and the explanatory view in FIG. 14. Color signals are sampled with sampling frequencies of 2 f.sub.sc and color signal amplitudes at the respective sampling points are normalized and are shown in Table 1.
TABLE 1 __________________________________________________________________________ Present signal -1 1 -1 1 -1 1 -1 1 -1 1H delayed signal 1 -1 1 -1 1 -1 1 -1 1 2H delayed signal -1 1 -1 1 -1 1 -1 1 -1 Color signal j -1 1 -1 1 -1 1 -1 1 -1 __________________________________________________________________________
TABLE 2 ______________________________________ Present 0 0 0 -1 1 0 0 signal d 1H delayed 0 0 1 -1 0 0 0 signal e 2H delayed 0 -1 1 0 0 0 0 signal f Middle value 0 0 1 -1 0 0 0 output h Adder 9 0 0 1 -1 1 0 0 output i Adder 8 0 -1 1 -1 0 0 0 output g Color 0 0 1 -1 0 0 0 signal j ______________________________________
As the color signal frequency is sufficiently higher than the horizontal frequency, the normalized color signal amplitude can be all considered to be 1. As shown in Table 1, the color signal is inverted in the phase before and after 1H.
Now, if the operation mid (A, B, C) is defined to be an operation to determine the middle value of A, B and C, the output h of the middle value detecting circuit 10 will be able to be represented by the below mentioned formula (2): EQU h=mid(d, e', f) (2)
wherein e' represents an inverted signal of the signal e. The color signal j from the middle value detecting circuit 15 can be represented by the below mentioned formula (3): EQU j=mid(h,(d+e')/2, (e'+f)/2) (3).
The color signal j in case it is input is determined by applying this formula (3) to the above mentioned Table 1. The signal e is made by inverting the 1H delayed signal in Table 1. The same output as of the present signal is obtained from the middle value detecting circuit 10. Further, the same color signal j as the present signal is obtained also from the middle value detecting circuit 15.
On the other hand, a black and white video signal having an oblique luminance component is assumed to be input through the input terminal 1. FIG. 14 shows a picture in which an image having such oblique component is displayed and the boundary between the black part (hatched part) and white part is sectioned by an oblique right rising varying line 20. FIGS. 15(a) to (f) are waveform views showing respectively the signals a to f in FIG. 13 in case the video signals in FIG. 14 are input. The above mentioned Table 2 shows signals d, e, f, h, i, g and j of respective parts in case the video signals in FIG. 14 are input as sampled by a sampling frequency of 2 f.sub.sc and having had the amplitude normalized.
As shown in FIGS. 15(a) to (c), the video signals of predetermined three lines lag by a predetermined period in the timing varying from a black level to a white level in response to the inclination of the varying line 20. These signals a, b and c are limited in the band respectively by the BPF's 4, 5 and 6 so as to be converted to the signals of the amplitudes shown in FIGS. 15(d) to (f). By the way, the circle (.largecircle.) marks in FIGS. 15(d) to (f) show sampling points for making the above mentioned Table 2.
These signals d, e and f are input into the middle value detecting circuit 10 to obtain the output h based on the above mentioned formula (2). The adders 8 and 9 output respectively the signals g and h in Table 2. The middle value detecting circuit 15 operates the above mentioned formula (3) and outputs the color signal j shown in Table 2. As shown in Table 2, a part in which the amplitude is not 0 is generated in the color signal j from the middle value detecting circuit 15. Thus, though the input video signal is black and white and essentially contains no color component, in case an oblique component is contained in the luminance signal, a color component will be output from the output terminal 18 and a cross color will be produced. For example, there has been a problem that such picture as of a neck tie of black and white oblique stripes is displayed with the addition of colors.